#include <bits/stdc++.h>

#define rep(i,n) for(int i=0;i<(n);i++)

using namespace std;
using lint=long long;

int main(){
	int n,m; scanf("%d%d",&n,&m);
	vector<lint> a(n);
	rep(i,n) scanf("%lld",&a[i]);
	vector<int> x(m);
	vector<lint> w(m);
	rep(j,m) scanf("%d%lld",&x[j],&w[j]), x[j]--;

	{
		vector<lint> tmp(n);
		rep(j,m) tmp[x[j]]+=w[j];
		x.clear();
		w.clear();
		rep(i,n) if(tmp[i]>0) {
			x.emplace_back(i);
			w.emplace_back(tmp[i]);
		}
		m=x.size();
/*
		vector<pair<int,lint>> p(m);
		rep(j,m) p[j]={x[j],w[j]};
		sort(p.begin(),p.end());
		rep(j,m) tie(x[j],w[j])=p[j];
		for(int j=m-2;j>=0;j--) if(x[j]==x[j+1]) {
			w[j]+=w[j+1];
			w[j+1]=0;
		}
		rep(j,m) if(w[j]==0) {
			swap(x[j],x[m-1]);
			swap(w[j],w[m-1]);
			j--;
			m--;
		}
		x.resize(m);
		w.resize(m);
*/
	}

	rep(j,m) if(w[j]>=a[x[j]]) return puts("-1"),0;

	if(accumulate(w.begin(),w.end(),0LL)<*min_element(a.begin(),a.end())) return puts("0"),0;

	const int OFFSET=2e5;
	vector<lint> sum(n+2*OFFSET+1);

	int lo=0,hi=1e5;
	while(hi-lo>1){
		int mi=(lo+hi)/2;

		int c=mi;
		fill(sum.begin(),sum.end(),0LL);
		rep(j,m){
			// left
			sum[x[j]-w[j]/c+OFFSET]+=c;
			sum[x[j]+OFFSET]-=c;
			sum[x[j]-w[j]/c+OFFSET-1]+=w[j]%c;
			sum[x[j]-w[j]/c+OFFSET]-=w[j]%c;
			// right
			sum[x[j]+OFFSET]-=c;
			sum[x[j]+w[j]/c+OFFSET]+=c;
			sum[x[j]+w[j]/c+OFFSET]-=w[j]%c;
			sum[x[j]+w[j]/c+OFFSET+1]+=w[j]%c;
		}
		rep(_,2) rep(i,(int)sum.size()-1) sum[i+1]+=sum[i];

		bool ok=true;
		rep(i,n) if(sum[i+OFFSET]>=a[i]) { ok=false; break; }
		if(ok) hi=mi;
		else   lo=mi;
	}
	printf("%d\n",hi);


	return 0;
}